1. Field of Invention
This invention relates generally to processes for forming aggregates, such as extrudates, containing zeolite for use as adsorbents and as substrates for chemical catalysts.
More specifically, the present invention is directed to producing aggregates which are "skin-free" i.e., essentially devoid of substance which interferes with communication between openings in the exterior surface of the aggregate, micropores of the zeolite bound in the aggregate, and mesopores or interstitial spaces within the aggregate communicating between these openings to the exterior surface of the aggregate and the micropores for the zeolite.
Related to this, the present invention is also directed to producing aggregates which have a crush strength greater than about 0.9 pounds per millimeter; and loss by attrition of less than about 3.0%. Catalysts based on such aggregates also exhibit catalyst activity pass through to the zeolite bound into the aggregate of at least about 70% of the catalyst activity of the freshly prepared zeolite prior to being bound therein.
The present invention is also directed to regenerable catalysts, such as reforming catalysts, which are composed of a catalyst metal dispersed in zeolite, bound by a binder composed of a metal oxide containing aluminum formed into such aggregate, wherein the catalyst exhibits a level of regenerability, expressed as a ratio of the catalyst activity test rating of the catalyst as regenerated relative to the catalyst activity test rating of the catalyst in a fresh state prior to operation on oil, of at least 70%.
The present invention is also directed to reforming processes which involve exposing a hydrocarbon stream under reforming condition to a regenerable catalyst, produced in accordance with the present invention, in addition to processes for purifying a hydrocarbon stream by contacting the hydrocarbon stream under conditions suitable for adsorption of contaminants from the hydrocarbon stream with an aggregate produced in accordance with the present invention.
2. Discussion of Background and Material Information
Since the advent of high compression automobile and aircraft gasoline engines in the late 1930s, the demand for high octane gasoline has risen continuously. Part of the octane requirement is satisfied by adding organo-lead compounds and oxygenated organic compounds to motor gasoline blends. Also, catalytic reforming, a major petroleum refining process is used to raise the octane rating of hydrocarbons, such as naphthas (C.sub.5 to C.sub.11 hydrocarbons), for gasoline blending. Catalytic reforming is also a principle source of industrially important light aromatic chemicals (benzene, toluene and xylenes) via conversion of paraffins and naphthenes to aromatics. The principle chemical reactions which occur during reforming are dehydrogenation of cyclohexane to aromatics, dehydrocyclization of paraffins to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to branched paraffins, dealkylation of alkylbenzenes, and cracking of paraffins to light hydrocarbons. The last reaction is undesirable since it produces light hydrocarbons which have low value. Also, coking and agglomeration of catalytic metals occur, which lead to deactivation of the catalyst over time.
Reforming is carried out at temperatures between about 800.degree. F. to about 1000.degree. F., pressures of about 50 to about 300 psi, weight hourly space velocities of 0.5 to 3.0, and in the presence of hydrogen at hydrogen to hydrocarbon molar ratios of 1 to 10. Commercial reforming units typically include a multiplicity of adiabatic packed bed reactors connected in series. Both axial and radial flow reactors are employed and these can be either stationary or moving bed reactors.
The hydrocarbon feed is vaporized, mixed with hydrogen and preheated in a furnace to about 800.degree. F. to 1000.degree. F. and fed into the inlet of the lead reactor. Reforming is a net endothermic process so the temperature of the reacting gas stream drops as the stream moves through the reactors, and reactor effluents are usually in the lower end of the 800.degree. F. to 1000.degree. F. reforming temperature range. Accordingly, reactor effluent streams are reheated in furnaces installed upstream of each of the reactors. The product stream from the tail reactor is cooled and flashed to low pressure in a drum and separated into a liquid reformate stream rich in aromatics, and a gas stream rich in hydrogen. Part of the hydrogen stream is recycled into the feed stream using a compressor to provide the hydrogen required for the process. Reforming is a net hydrogen producing process. The net hydrogen produced in the process is removed as a gas stream from the flash drum, which is recovered and purified.
Currently, the most widely used commercial reforming catalysts are comprised of a Group VIII metal such as platinum or platinum plus a second catalytic metal such as rhenium or iridium, dispersed on an alumina substrate. These catalysts are bifunctional; that is they have two types of catalytic sites: metal sites and separate strong acid sites. Typically, chlorine is incorporated on the alumina to add strong acid site functionality. These catalysts accomplish dehydrogenation and cyclization reactions on the metal sites and the isomerization reactions on the strong acid sites. Cracking reactions, which are undesirable because they convert feed to low value gases, occur primarily on the acid sites. These alumina based bifunctional reforming catalysts are effective for aromatizing C8+paraffins but are less effective for aromatizing C6 to C8 paraffins; they crack more of the lighter paraffins to low value fuel gas than they convert to aromatics.
Within the past few years new reforming catalysts have been discovered which are more effective for aromatizing the C6 to C8 paraffin components of naphthas. These new catalysts employ large pore zeolites to support the catalytic metal. The zeolite catalysts are mono-functional; they contain few strong acid sites. They accomplish the isomerization, as well as dehydrogenation reactions on the metallic catalytic sites with facility. Unwanted cracking reactions are repressed because there are few strong acid sites in the catalysts.
Large pore zeolites, i.e., zeolites with effective pore diameters of 6 to 15 Angstroms, are preferred for reforming catalysts. Suitable large pore zeolites include zeolite X, zeolite Y, and zeolite L. The preferred large pore zeolite for reforming catalysts is zeolite L which is described in detail in U.S. Pat. No. 3,216,789, hereby incorporated by reference thereto herein.
U.S. Pat. Nos. 4,104,320, 4,416,806 and 4,417,083 disclose the use of zeolite L as substrates for reforming catalysts. Specific morphological forms of zeolite L which convert to superior reforming catalysts are disclosed in U.S. Pat. Nos. 4,552,856, and 4,544,539.
Ideally, reforming catalysts should i) display high activity and selectivity to aromatics and isoparaffins; ii) be regenerable; iii) survive a cost effective number of regenerations; iv) possess sufficient crush strength and loss by attrition to avoid excessive breakdown in reactors (because pressure drop across commercial reactors can become unacceptably high if the amount of catalyst fines in the catalyst becomes excessive); and v) be sufficiently cost effective not to add unreasonably to the cost of operation.
Reforming catalysts containing platinum, with or without the addition of other promoter metals such as rhenium, have been used for some time. These metals are often supported on alumina.
Catalysts containing type L zeolite have been discovered to be useful for catalytic dewaxing and in other applications. They are also particularly useful in reforming because they are effective for aromatizing C.sub.6-C.sub.8 paraffin components and crack less feed to gas relative to conventional catalysts. In this regard, U.S. Pat. Nos. 4,104,320; 4,417,083; 4,416,806 and British Application 2106413A, Bernard et al., disclose the use of zeolite L as a support which increases the selectivity of the reaction for producing aromatic products and also disclose processes for using the zeolite L and methods for its regeneration.
Catalysts of platinum on potassium type L zeolites have been disclosed in U.S. Pat. Nos. 4,552,856, TAUSTER et al., and U.S. Pat. No. 4,544,539, WORTEL, the latter of which is directed to an improved cylindrical zeolite L aromatization catalyst.
Zeolites are synthesized as microcrystals typically 5 to 20 thousand angstroms in size. To be suitable for use in commercial packed bed reactors, zeolites in their natural fine powder state must be formed into aggregates, such as aggregated particles, for example, tablets, spheres, prills, pills or extrudates, typically 1/32 to 1/8 of an inch in size. If zeolite powder were to be charged to the reactors as synthesized, pressure drop across the catalyst beds at commercially viable feed rates would be impractically high. Commonly, inorganic oxides such as alumina, silica, alumino-silicates and clays are used as binders to hold the aggregates together. The aggregates must have sufficient crush and loss by attrition so that they do not disintegrate in the packed bed reactors under normal commercial operating conditions but also the zeolite should retain an effective level of the catalytic activity it exhibits in the unbounded form and the binder should not add undesirable chemical activity to the catalyst's functionality, i.e., a combination of attributes most difficult to accomplish.
In the production of type L zeolites reforming catalyst, it is known in the art to use alumina as a binder or support. For example, U.S. Pat. Nos. 4,458,025, Lee et al., 4,517,306, Buss, and its divisional 4,447,316 both make such a suggestion. The disclosure in U.S. Pat. No. 4,458,025 suggests extrusion of a type L zeolite in alumina.
Related to this, U.S. Pat. No. 3,326,818, GLADROW et al., disclose a catalyst composition made up of a crystalline aluminosilicate and a binder prepared by mixing the crystalline aluminosilicate in a minor amount of dry inorganic gel binding agent, such as alumina. The alumina is disclosed as containing a minor amount of a peptizing agent for the purpose of enhancing the strength of the resulting product.
U.S. Pat. No. 3,557,024, Young et al., disclose alumina bound catalysts having a composition formed by mixing one of a number of zeolites, including zeolite L, with a binder of hydrous boehmitic alumina acidified with at least 0.5 mole equivalent of a strong acid per mole of alumina. The catalyst is disclosed as having enhanced strength.
U.S. Pat. No. 4,046,713, MITSCHE et al., disclose a method for preparing an extruded catalyst composition wherein acidic alumina hydrosol is admixed with a dry mixture of a finely divided alumina, preferably a hydrate, and a finely divided crystalline aluminosilicate, such as mordenite. The resulting mixture is extruded, dried and calcined to form a catalyst disclosed as being useful in the reforming of various naphthas. The aluminosilicate makes up less than 20% of the mixture.
U.S. Pat. Nos. 4,305,812; 4,305,811;4,306,963; and 4,311,582, JOHNSON and JOHNSON et al., are directed to stabilized reforming catalysts which are halide promoted. Each of the catalysts is produced by employing a modified alumina support whose alumina precursor includes at least about 75% by weight boehmite.
LEE and SANTILLI in U.S. Pat. Nos. 4,458,025 and FIELD, 4,579,831, disclose a process for making a zeolite L catalyst by mulling a non-acidic alumina sol with zeolite L and extruding the resulting paste.
In LEE and SANTILLI, U.S. Pat. No. 4,458,025, it is disclosed that after the alumina is peptized with acid to form a sol, the resultant alumina sol is back-neutralized to neutrality. Where a non-acidic alumina sol is peptized with a base, no back-neutralization and wash is needed. After extrusion, the extrudate is dried and calcined at about 1,000.degree. F. for about 1-2 hours.
In U.S. Pat. No. 4,579,831, FIELD, the alumina used as the binder contains either an alkali or an alkaline earth component. The catalyst is disclosed as being formed by forming a solution of an alkali metal aluminate then adjusting the pH of the solution to a pH of from 6 to 8, and aging the solution. The aluminate solution is then filtered, dried and mulled with a large-pore zeolite to form a mixture which is extruded to form an extrudate which is dried, calcined and impregnated or exchanged with a Group VIII metal to form a catalyst which is subsequently dried and calcined. It is disclosed that the extrudate is dried and calcined to add strength to the resultant catalyst and should be conducted in a first step of about 1100.degree. F. for about 2 hours. Once the extrudate has been calcined and impregnated with the Group VIII metal to form a catalyst, the catalyst is then dried and subjected to a second calcination at a temperature of about 500.degree. F., instead of the 1000.degree. F. used in the first calcination.
TROWBRIDGE in pending U.S. Pat. application Ser. No. 06/880,087 (now abandoned) teaches extruding zeolites using a two-component alumina binder comprised of boehmitic alumina and an acidic alumina sol, the disclosure of which is hereby incorporated in its entirety by reference thereto.
Conventional zeolite extrudates used as catalyst substrates, however, tend to have a relatively thick outer skin of alumina, and a coating of alumina, surrounding the zeolite crystals which inhibits catalyst activity. In addition, conventional alumina bound zeolite X aggregates contain residual acidity which induces cracking and impairs activity and selectivity maintenance under reactive conditions.